You are wrong about weight of the rider. https://www.helpfulhorsehints.com/right-size-horse-for-your-height-and-weight/
I grew up riding. We had a trained 16.5 hand hunter/jumper that my father and I both rode on together comfortably. He was around 240-265 of pure muscle. I was 4-10yrs old during the time so less than 100lbs. I have also done some stockyard work and seen some fatties work a large horse without any issues. EDIT: like the post regardless :-)
[–] pepeshadilay ago (edited ago)
https://www.horsetalk.co.nz/2013/03/26/many-riders-too-heavy-for-mounts/
Riding horses weigh around 1200 pounds on average; 20% is a safe weight limit. Thus a 1200-lb horse can carry a 240-lb rider. (Icelandic ponies can carry 30% of their weight, but they are hard to find and they don't weigh much.)
Medieval knights in heavy armor presumably had very heavy horses, and note that most knights did not wear full armor when they rode their everyday horses. Many knights had a heavy warhorse only for battle, and a light riding horse for normal riding. (Of course medieval Europe is a huge topic and I am just scratching the surface.)
[–] Derpfroot ago
I rode a bit when I was younger and loved it. We rode out, ate lunch by a creek (saw a snake eating a mouse) and came back...so fun. I've actually been really wanting to lately since I remembered that...well, that and the boy's been asking to go. Now I have to go.